Isolated soy proteins (ISP) contain from 17 to 33 parts per million (ppm) free sulfites which are being formed after the isoelectric precipitation step of ISP processing (Stine, Boatright and Lu, 2004). Soybeans contain a relatively high number of transition metals, including manganese (Waggle and Kolar, 1979). When sulfites, manganese and oxygen are combined in solution, sulfite-free radicals are generated (Moya and other, 1999). Further reaction of the sulfite-free radicals with oxygen can produce sulfate-free radicals (Ermakov and Purmal, 2002). This spontaneous reaction involving sulfite, manganese and oxygen, appears not to have been previously documented in any human food. Sulfite- and sulfate-free radicals can contribute to the degradation of the essential amino acids in proteins (Ido and Kwanishi, 1991), which can alter the functional and nutritional characteristics of the protein.
Because there is no previously reported occurrence of sulfite- or sulfate-free radicals in foods, there are few studies on the toxicity of these powerful oxidants to humans. Among the studies that do exist, sulfite-free radicals have been linked to DNA damage, cancer (Shi and Mao, 1994; Jameton and others, 2002), and atherogenesis (Laggner and others, 2005). Also, because sulfate-free radicals are very reactive toward the oxidation of methionine residues, they may also play a role in the development of Alzheimer's disease and Parkinson disease (Choi and others, 2006; Clementi and others, 2006).
Boatright and others (2006) found that the sulfite content of ISP is elevated during ISP processing, with a rather large increase occurring after the protein is precipitated and separated from whey (soluble components at pH 4.5) during isoelectric precipitation. The present invention is directed to treating ISP with cystine or iodate to reduce the level of sulfite, sulfite free-radicals, sulfate free radicals and other free radicals generated from sulfite free radicals, and methanethiol in ISP.
The use of iodate to treat textured soy protein (TSP) was described in U.S. Pat. No. 4,259,364 to Chung (1981). The patent discloses preparing a textured soy protein by a thermoplastic extrusion process and subsequently washing the TSP with an iodate solution to remove oxidized flavor precursors.
TSP is very different from ISP. Sulfites have not been reported in TSP. Chung proposed that iodate be added to TSP to oxidize the precursors of flavor compounds.
In the present invention, the addition of iodate or cystine during the processing of ISP inhibits sulfite and the subsequent formation of sulfite free-radicals, sulfate free radicals and other free radicals generated from sulfite free radicals, and methanethiol. Iodate oxidizes the sulfites to sulfates. Cystine reacts with free sulfites to form cysteine-S-sulfonates. Transition metals such as manganese, ferrous iron, ferric iron and copper are found in soy proteins (Waggle and Kolar, 1979). The free radicals resulting from the reaction between, for example, manganese and sulfites are involved in the degradation of methionine to produce methanethiol.